The present invention relates to an engine of the type that switches the combustion mode, and, more particularly, to a control device and control method that control an engine in accordance with a load acting on the engine.
In an ordinary vehicular engine, air drawn into a combustion chamber via an intake passage and fuel to be injected from a fuel injection valve are blended to form an air-fuel mixture. The engine acquires drive power by burning the air-fuel mixture in the combustion chambers. A throttle valve for regulating the amount of air to be led into the combustion chambers is provided in the intake passage. As the amount of air to be led into the combustion chambers is regulated by adjusting the degree of opening of the throttle valve, the amount of the air-fuel mixture to be filled in the combustion chambers changes, thereby adjusting the power of the engine.
Recently, engines of the type that switch the combustion mode in accordance with the running state of the engine have been proposed and made into practical use to improve fuel efficiency and to produce sufficient engine power at the same time. Such an engine is disclosed in, for example, Japanese Unexamined Patent Publication (KOKAI) No. Hei 5-288098.
The engine disclosed in the publication has a fuel injection valve that directly injects fuel into the combustion chambers. In high engine speed mode or high load mode, combustion takes place with fuel evenly mixed with air to produce sufficient engine power. Such a combustion mode is called homogeneous charge combustion. To execute homogeneous charge combustion, fuel is injected into a combustion chamber in the suction stroke of the engine. The injected fuel is evenly mixed with air in the combustion chamber and the homogeneous mixture of air and fuel is ignited by an ignition plug.
In low engine speed mode or low load mode, on the other hand, stratified charge combustion is executed in order to improve the fuel efficiency. In stratified charge combustion mode, the fuel concentration around the ignition plug is increased to improve ignition, and combustion is carried out while the average air-fuel ratio of the mixture in the combustion chamber is greater than the stoichiometric air-fuel ratio. To execute stratified charge combustion, fuel is injected into the combustion chamber in the compression stroke of the engine. The injected fuel strikes a dent provided at the top of the piston and is gathered around the ignition plug. The mixture of the gathered fuel and the air in the combustion chamber is ignited by the ignition plug.
In stratified charge combustion, the degree of opening of the throttle valve is greater than that in homogeneous charge combustion mode. As a result, the pumping loss is reduced.
As the combustion mode for an engine is switched between homogeneous charge combustion and stratified charge combustion in accordance with the running state of the engine in the manner described above, the fuel efficiency can be improved and sufficient engine power is produced.
Normally, engines are controlled in accordance with the load. One example of control according to the engine load is fuel injection amount control. For the engine in which the combustion mode is switched, a parameter representing the amount of intake air, e.g., the amount of intake air itself or the intake pressure, is used as a value representing the engine load in homogeneous charge combustion mode. In accordance with the value of that parameter, the fuel injection amount is controlled.
In stratified charge combustion, the throttle angle is greater than that in homogeneous charge combustion mode. If a parameter representing the amount of intake air is used to control the fuel injection amount in stratified charge combustion mode, the fuel injection amount is inadequate for the engine load. In stratified charge combustion mode, therefore, the amount of manipulation of the acceleration is used to represent the engine load and the fuel injection amount is controlled in accordance with the position of the acceleration pedal.
As apparent from the above, fuel injection amount control according to the engine load is executed adequately by switching the value that is used to represent the engine load, in accordance with the combustion mode of the engine.
That the value to be used to represent the engine load differs in accordance with the combustion mode of the engine means that controls executed in accordance with the engine load in homogeneous charge combustion and stratified charge combustion are independent.
The control that is performed in accordance with the engine load generally affects the power torque of the engine. If controls that take place in accordance with the engine load in homogeneous charge combustion and stratified charge combustion are independent, however, it is difficult to match the engine power torque characteristics between the combustion modes. In particular, fuel injection amount control significantly influences the engine power torque characteristics including the response characteristic in the transient state of the engine power torque characteristics. It therefore is difficult to match the engine power torque characteristics between homogeneous charge combustion and stratified charge combustion.
The present invention has been made in view of such situation, and the object of the present invention is to provide a control device and control method for an engine that can easily match the engine output torque characteristics between homogeneous charge combustion and stratified charge combustion during engine control that depends on engine load.
To achieve the object, the present invention provides a control device for an engine that acquires power by burning a mixture of air and fuel in a combustion chamber. The engine has an acceleration pedal and a throttle valve for adjusting an amount of intake air to the combustion chamber. The engine switches a combustion mode between homogeneous charge combustion and stratified charge combustion. The control device has control means that controls the engine in accordance with a load acting on the engine. When homogeneous charge combustion is executed, the control means uses a parameter representing the amount of intake air as a value representing the engine load. The control device further has computation means for computing, as a virtual parameter, a value equivalent to the parameter presuming homogeneous charge combustion is executed with an amount of manipulation of the acceleration pedal when executing stratified charge combustion. When stratified charge combustion is executed, the control means uses the virtual parameter as a value representing the engine load.
The present invention also provides a control method for an engine that acquires power by burning a mixture of air and fuel in a combustion chamber. The engine has an acceleration pedal and a throttle valve for adjusting an amount of intake air to the combustion chamber. The engine switches a combustion mode between homogeneous charge combustion and stratified charge combustion. The control method includes the steps of controlling the engine in accordance with a load acting on the engine; using a parameter representing the amount of intake air as a value representing an engine load when homogeneous charge combustion is executed; computing, as a virtual parameter, a value equivalent to the parameter presuming homogeneous charge combustion is executed with an amount of manipulation of the acceleration pedal when executing stratified charge combustion; and using the virtual parameter as a value representing the engine load when stratified charge combustion is executed.
In either combustion mode, homogeneous charge combustion or stratified charge combustion, a common parameter representing the amount of intake air is used as a value representing the engine load in controlling the engine. This associates engine control according to the engine load in homogeneous charge combustion with that in stratified charge combustion, thus ensuring easy matching of the engine output torque characteristics between the combustion modes.